In optical systems, in-line devices such as diffraction gratings and prisms have heretofore been used as transmission or reflection wavelength filters. Alternatively, devices using the interferometric principles of cavity resonators, such as, for example, the Fabry-Perot etalon have been proposed as wavelength filters or selective amplifiers. An example of such an interferometric filter is disclosed in published European Patent Application EP 143645, in which the ends of two optical fibres are mirrored and longitudinally aligned in close proximity in a suitable housing. The arrangment provides an etalon which forms a wavelength filter that can be adjusted by varying the separation of the fibre ends. Problems which can arise with `in-line` resonators of this kind are the losses which are introduced, for example, owing to electromagnetic mode mismatch, spurious reflection and other factors, as a consequence of the break in the continuity of the transmission line.
An alternative approach to providing an optical filter comprises using a ring of optical fibre, as described, for example, by Stokes et al. in "All-single-mode fiber resonator", Optics Letters, Vol. 7, No. 6, June 1982. Stokes' device consists of a loop of optical fibre 3 m in circumference. For most purposes the spectral response of a ring of such length is excessively narrow, and a device of this size would clearly be impractical for most applications. However, reducing the size by using rings of smaller circumference, and thereby broadening the spectral response, has been found to introduce curvature dependent bending losses which increasingly degrade the overall performance as the radius of curvature of a ring is reduced.
It is an object of the present invention to provide an improved, selective optical cavity resonator which avoids, or at least mitigates, some of the aforementioned problems.
According to the present invention a dielectric optical waveguide device comprises a first waveguide transversely coupled to a second waveguide at a coupling region, wherein one end of each waveguide selected at opposite ends of said coupling region is provided with a reflection means, thereby to form a Fabry-Perot cavity.
Preferably, both the waveguides are optical fibres; most preferably, monomode optical fibres.
The waveguides may alternatively be fabricated on substrates, such as, for example, lithium niobate.
Conveniently, the reflection means comprise reflective surfaces provided by deposition directly on the waveguide ends.
The reflective surfaces may comprise metallic coatings, and may conveniently be provided by conventional techniques such as evaporated gold/aluminium deposition or by grown silver. Alternatively, the reflective surfaces may be dielectric reflection coatings.
In place of direct deposition, the reflection means may be provided by mirrors butt-mounted on the waveguide ends. As a further alternative, the reflection means may comprise other apparatus or combinations of apparatus known to the skilled worker and adapted to perform the function of optically reflecting incident light over the operating spectral range of the device.
When the device is employed as an optical filter, preferably the reflection means will be very highly reflective. Over the operating spectral range of the device the reflectivities will be preferably at least 0.8, and more preferably, at least 0.9.
Conveniently, the device is tunable, and further comprises means to effect the tuning. For example, tuning may be achieved by altering the effective length of the Fabry-Perot cavity. In one embodiment, the tuning means comprises a piezo-electric stretcher to adjust the length of one of the waveguides where that waveguide forms part of the Fabry-Perot cavity.
Either or both waveguides may incorporate a dopant, in addition or alternative to conventional refractive index modifying dopants, to alter the optical properties thereof. The dopant may comprise a light amplifying medium, such as, for example, neodymium. In these circumstances, lower reflectivity reflection means may be desirable.